1951 Bowman Mantle is a stunner and very undervalued. I think the 56 Mantle is his nicest looking card and being from the Triple Crown year is a bonus.
The 1951 Bowman Mays, 1954 Topps Aaron are pretty sweet as well. Ryan RC is great card but overrated IMO. I like the 63 Rose as well.
If we are going all sport post war I would have to vote the 1986-87 Fleer Jordan as a must have for any collection. Most iconic post war sports card after the Mantle.